1. Field of the Invention
The present invention relates to an AM/FM receiver, and more specifically, to a circuit for detecting the level of noise in a received frequency-modulated (FM) signal, which can be used as an FM signal strength detection circuit in an AM/FM receiver which includes at least an FM intermediate-frequency amplifier, an FM detector connected thereto, an AM intermediate-frequency amplifier and an AM detector connected thereto.
2. Description of the Related Art
Conventionally, in various broadcast receivers such as AM (amplitude modulation) broadcast receivers, FM (frequency modulation) broadcast receivers, AM/FM broadcast receivers, television broadcast receivers, etc., there are used different types of circuits for detecting the level or the field strength of the received signal, for automatic control of various parameters of the receivers such as gain, tuning, etc.
In AM receivers of the superheterodyne type, the output current from an AM detector is representative of the level of a received AM signal, and therefore, the gain of an intermediate-frequency (I.F.) amplifier is ordinarily controlled by feeding back to the intermediate-frequency amplifier a portion of the output current from the AM detector for the purpose of automatic gain or volume control. In certain cases, a portion of the AM detector output is utilized for tuning indication. In any case, the level of the received signal can be detected without adding a substantial circuit to the fundamental circuits indispendable for reception of AM broadcasting.
On the other hand, in FM receivers, it has been considered that the output of an FM detector does not represent the level of a received FM signal because the amplitude of the signal demodulated from the received FM signal is substantially independent upon the amplitude of the received FM signal per se. In order to detect the level of the received FM signal, therefore, a FM signal is sampled from a suitable stage of the FM intermediate-frequency amplifier and rectified by a special circuit to provide a DC voltage representative of the level of the received FM signal. However, since a substantial additional circuit is needed for such a signal processing, the FM receivers had to comprise an increased number of circuit elements and to inevitably become more expensive. Particularly, in the case of detecting the level of the received FM signal by sampling a FM signal from the output of each stage of the FM intermediate-frequency amplifier, rectifying each sampled signal and summing up all the rectified sampled signals, such a signal processing have needed an additional circuit composed of about one half of the intermediate-frequency processing circuit in the number of circuit elements, and therefore, the FM receiver had been at a substantially increased cost.